Climate change, impacts and vulnerability in Europe ... - MemoFin.fr

Changes in the climate system2.2.5 Precipitation extremesRelevanceChanges in the frequency and intensity of extremeprecipitation (see Box 2.4 for definitions) can haveconsiderable impacts on society, including the builtenvironment, agriculture, industry and ecosystemservices. An assessment of past trends and futureprojections of extreme precipitation is thereforeessential for advising policy decisions on mitigationand adaptation to climate change (Kendon et al.,2008). The risks posed by precipitation-relatedhazards, such as flooding events (including flashfloods) and landslides, are also influenced bynon‐climatic factors, such as population density,floodplain development and land-use change.Hence, estimates of future changes in such risksneed to consider changes in both climatic andnon‐climatic factors. Estimates of trends in heavy orextreme precipitation are more uncertain than trendsin mean precipitation because, by their very nature,extreme precipitation events have a low frequency ofoccurrence. This leads to greater uncertainties whenassessing the statistical significance of observedchanges.Past trendsObservational records do not indicate widespreadsignificant trends in either the number ofconsecutive wet days (indicating flood risks) ordry days (indicating drought risks) across Europe(Map 2.7). Some changes in these variables havebeen observed across Europe but most of themare not statistically significant due to large naturalvariability. Interestingly, parts of north-western andnorth-eastern Europe show significant increasingtrends in both the number of wet days and dry days.The proportion of Europe that has experiencedextreme or moderate meteorological droughtconditions did not change significantly during the20th century (Lloyd-Hughes and Saunders, 2002).Summer droughts have also shown no statisticallysignificant trend during the period 1901–2002(Robock et al., 2005) ( 41 ).Box 2.4Definition of precipitation extremesThe term 'precipitation extreme' can refer to both high and low extremes of precipitation. High extremes ofprecipitation are defined either by the amount of precipitation in a given time period (e.g. daily or hourly) or bythe period above a location-specific threshold. Indicators for extreme precipitation may be defined in terms ofreturn periods (such as 1 in 20 years, termed absolute extremes). Alternatively, they may be defined relativeto the normal precipitation distribution in an area (e.g. the heaviest 5 % of daily precipitation averaged over a30‐year time period, relative extremes). The former captures infrequent but possibly high impact events, whereasthe latter has a more frequent sampling rate, capturing on average 18 days per year (over 30 years). Theapproach that is chosen depends largely on the application of the end user.Low extremes of precipitation can be quantified in terms of consecutive dry days. Note, however, that anassessment of changes in drought conditions should also consider other factors, such as soil moisture andvegetation responses to changing atmospheric CO 2concentration.Key messages: 2.2.5 Precipitation extremes• There are no widespread significant trends in either the number of consecutive dry or wet days acrossEurope.• Heavy precipitation events are likely to become more frequent in most parts of Europe. The changes arestrongest in Scandinavia in winter and in northern and eastern central Europe in summer.( 41 ) More about droughts can be found in Section 3.3.4.68 Climate change, impacts and vulnerability in Europe 2012